Really need help

I just installed my new PSU a seasonic S12 620W threw out my hd5450 and put in my rx 480 and nothing shows up on screen it's just rebooting after like 30 seconds for one time every time I start it manually

Please post all your specs - motherboard, CPU, cables you used to connect PSU to GPU, etc.
What have you tried so far to solve the issue?
Have you tried reseating the card?
Have you tried the card with another PSU and the PSU with another card?